When paper sheets such as bills are distributed in the market, it is necessary to only and appropriately distribute bills that are not stained and the like. Accordingly, bills that should be distributed and bills that should not be distributed in the market need to be accurately discriminated, and the bills that should not be distributed need to be collected.
The discrimination between the bills that should be distributed, and the bills to be collected and not to be distributed have been made based on the presence of a tear in the bill, the amount of stain on the printed area of the bill, and the like. However, even if the amount of stain on the entire printed area of the bill is small, some bills should not be distributed in the market. For example, a person may be accused of lese majeste, if he or she scribbles on a portrait portion and the like on the bill in the countries such as Thailand, Malaysia, and Singapore. Accordingly, it has been desired to strictly discriminate the presence of a stain on a predetermined printed area such as a portrait.
A bill serial number reading unit disclosed in Patent document 1 reads a portion where the serial number of the bill is printed, and recognizes a character of each digit of the serial number from image data of the serial number area. The bill serial number reading unit also replaces an unrecognizable character with an error character that indicates a recognition error, adds the error character to the recognized characters, and outputs the recognition result.    Patent document 1: Japanese Patent Application Laid-open No. 2004-213559